We are surrounded with EMI, electromagnetic interference.
What you observe on the scope or hear on an audio amplifier is this EMI, particularly AC line frequency and its harmonics.
A human body alive or dead acts as a pickup antenna.

The human body does generate its own electrical signals. If your probes are in the right spot, you can read them. Google EMG signals. But if you aren't deliberately looking for those signals then you're probably experiencing EMI as mrchips said.